This invention relates generally to crude oil desalter/dehydrator vessels and, more specifically, to desalter/dehydrator vessels which utilize a set of electrodes.
A common crude oil desalter/dehydrator used by many of the world's refiners was invented by Petreco in 1980 and is illustrated and described in U.S. Pat. Nos. 4,149,958; 4,182,672; 4,188,277 and 4,209,374 (“the Petreco patents.”) Desalters/dehydrators marketed under the Petreco patents have been referred to as BILECTRIC® desalters (hereinafter “bilectric desalter”). A bilectric desalter utilizes a set of two or three parallel electrodes arranged horizontally in a horizontal vessel. After the crude oil containing salt is prepared by adding fresh water, the oil-water mixture is introduced horizontally into the vessel and between these electrodes. This technique of desalting coalesces the dispersed water in an intense electrostatic field and has the advantage of achieving rapid and highly efficient droplet coalescence and separation.
Historically, a bilectric desalter utilizes two or three AC power units (transformers) designed to operate at a single voltage. Each power unit is connected to a separate, horizontally oriented electrode. The power unit on the lowest electrode establishes an AC electrostatic field between the oil/water interface and the energized electrode. This lowest electrode is intended to provide sufficient voltage to diminish and control the formation of a “rag” layer at the oil/water interface. If the applied voltage is not an effective one, the interface rag can accumulate which, in turn, places a greater power demand on the power unit. This effectively reduces the applied voltage and results in a less effective process to control the rag.
The middle (or upper) electrode establishes an electrostatic field between itself and the lowest electrode. This electrostatic field remains effective unless the rag layer intrudes into the zone between the electrodes and compromises the applied voltage. Finally, the upper electrode establishes an electrostatic field between the middle electrode and itself. The performance of this field is rarely compromised by the formation of an interface rag.
When an interface rag consisting of an unresolved oil/water emulsion accumulates on the oil/water interface, the interface rage hinders the strength of the electrostatic field which, in turn, leads to water chaining and shorting of the electrodes. To avoid the detrimental effects of water chaining, the AC voltage may need to be reduced. This reduction in voltage further compromises the bilectric desalter's ability to promote decay of the rag layer. The strength of the AC electrostatic field also limits the entrained water content to no more than 10%. When the water content increases above 10% the water droplets chain together and promote a shorting of the electrostatic fields.
As crudes become heavier and more difficult to desalt and dehydrate, they tend to form rag layers that are more stable and less likely to collapse in the presence of an electrostatic field. These “rag-producing” oils compromise the overall dehydration and desalting performance of the bilectric desalter.